In the field of drilling it is frequently desirable to communicate with devices which are located at the downhole end of the drilling assembly. There are few variable parameters which are readily transferable from the surface to the downhole location or assembly and all of these suffer from shortcomings. Largely, the measurable variables in the drilling operation are; the flow of fluids through the drillstring, the amount of weight which is placed on the bit and the revolutions of the drillpipe.
This disclosure acknowledges that weight on bit and fluid cycling are limited in their range of data transmission as, inevitably, they are confined to being binary input parameters. These surface variable parameters can have a negative impact upon the drilling operation when used as means of communicating with downhole devices, as; either the transmission time is lengthened which serves to interrupt the process of drilling the well or the data to be transmitted is, of necessity, reduced in content.
Previous attempts to communicate via drillstring RPM were successful but compromised the efficiency of the drilling operation in that the frequencies of operation were recurrently related to a baseline of zero RPM. In rotary drilling zero RPM equates to a non-drilling state, in other words, in order to be able to communicate using RPM the drilling operation had to be arrested, resulting in poorer drilling productivity and less rewarding economics. The essence of the instant invention is that it allows the baseline incremental drillstring RPM to be established and then increases or decreases RPM transmission in order to create a carrier for the desired data to be transmitted, without arresting the drilling operation. Expressed differently the instant invention uses the nominal drillstring RPM to establish itself as a carrier and then deviates from this established norm by marginal amounts. Assuming that the nominal RPM has been established in order to optimize drilling efficiency, the instant method and apparatus thus represents the best opportunity for adaptive downlink telemetry with the least interference to optimized drilling parameters. Yet a further benefit of the invention is the amount of data which may be transmitted in a timely manner from the surface of the earth to a downhole device or devices located at the distal end of the drilling assembly.
It is an axiom of rotary drilling that if a single revolution of the drillstring is input at the surface then it must be transmitted to the bit. Failure of the revolution to “transit” to the bit means either a “back-off” (the drillstring unscrewed) or a “twist-off” (the drillstring broke in two).
In the past, the reason for the use of zero RPM as a marker is that it has a definitive null value, either of vibration, rotation or rate of rotation and is therefore an easily measurable state.
Prior art [ENGELDER, U.S. Pat. No. 4,763,258] METHOD AND APPARATUS FOR TELEMETRY WHILE DRILLING BY CHANGING DRILLSTRING ROTATION ANGLE OR SPEED contemplated the use of solid state sensors which monitored “angularly dependent geophysical parameters while rotating the drillstring” in order to communicate from the surface to the downhole device. Magnetometers and inclinometers were sampled and signals therefrom were conditioned, multiplexed, converted to digital signals and then processed. By alternating the RPM with zero RPM bands and by altering the RPM ranges, information could be communicated to the downhole device. The device was limited in that the processing power and sensor sample frequency which was available at that time was much slower than that which is available at the present time. The device required slow rotation of the drillstring in order to communicate from the surface of the wellbore to the downhole device. Although this methodology is feasible, the length of the drillstring, directional characteristics of the wellbore and physical attributes of the drillstring are all variables which will all affect the ability to accurately transfer information to the distal end of the drilling assembly, or, more specifically, to determine with any degree of accuracy, the ‘arrival-time’ of the information at the distal end of the drilling assembly. A further difficulty with this particular arrangement, as previously explored, is the requirement to stop the drilling process, which, in practice, necessitates lifting the bit from the bottom-of the wellbore resulting in additional lost productive time. This is particularly required when drilling using aggressive, high torque, PDC bits, due to the resultant amount of on-bottom torsional friction which is created.
More recent prior art [MOUGEL AND HUTIN G.B. 2,352,743, U.S. Pat. No. 6,267,185] APPARATUS AND METHOD FOR COMMUNICATION WITH DOWNHOLE EQUIPMENT USING DRILLSTRING ROTATION AND GYROSCOPIC SENSORS removed the requirement for the measurement of geophysical parameters, substituting the measurement of non-geophysical parameters in the form of inertial rate gyroscopes. This, later art, taking advantage of faster downhole processor times, also claimed the possibility of both binary and decimal communication modes. The removal of dependent geophysical parameters would be of particular use when communications with downhole devices are planned in a zone of magnetic interference or in operational usage where there are unpredictable results from conventional geomagnetic sensors such as in surface conductor drilling beneath offshore platforms.
Additional prior art [van STEENWYCK et al. U.S. Pat. No. 6,608,565] DOWNWARD COMMUNICATION IN A BOREHOLE THROUGH DRILLSTRING ROTARY MODULATION concluded that additional transmitted data density could be achieved by modulating RPM either between a base level of zero RPM and a certain pre-determined value of RPM or, alternately, by eliminating the zero RPM baseline indicator, between two pre-determined values of RPM, which would potentially allow drilling to continue during drillstring rotary modulation.
U.S. Pat. No. 6,608,565 [van STEENWYCK et al.] proposes that two levels of modulation input are utilized to create “talkdown” waveforms. Talkdown is essentially a phrase describing information passed down to the distal end of the drillstring—“talkdown.” Relative pre-determined discrete rotation rates, (R1, R2) (“RPM”) are measured downhole against time and the default device for talkdown is described as an MWD device. This invention, applies a well understood measurement-while drilling (“MWD”) form of binary encoding technique and methodology to the transmission of data from surface to downhole.
The specification provides illustration and constraint on the method in FIG. 1 and FIG. 10, while the methodology of signal conditioning, processing and threshold identification and message capture of the downhole device is illustrated in FIGS. 5 through 7.
The data is preceded by a “sync” word consisting of a pulse width with a rising edge, corresponding to an increase in RPM, a pulse of equal width which corresponds to a decrease in RPM with a message word which consists of two periods of increased RPM, with a single band of lower RPM between. This format is considered to constitute optimal transmission methodology with minimal disruption to the drillstring.
In the field of drilling and in particular directional drilling, there is found a phenomenon known as “stick-slip” which is caused by a variety of friction factors of the drillstring rotating within the borehole. “Stick-slip causes the tubulars which comprise the drilling assembly (drillstring) to react like a coiled spring—winding up and unwinding: the degree and severity of the acceleration and deceleration of the drillstring, when compared with a nominal baseline RPM determines the classification of the qualitative condition which can be largely described as being anything from “mild” to “severe.” “Stick-slip” of whatever nature is not a desirable by-product, either from the perspective of drilling dynamics and efficiency, nor from the negative affect which it has on drilling tools which are located in the lower component of the BHA.
Historically, it is evident that stick-slip is an element which is difficult to quantify. It is almost impossible to avoid or eradicate during normal rotary drilling. It is the intention of this disclosure to introduce a system which is capable of surface power input management which may serve to reduce some of the peak accelerations which are observed at the distal end of the drillstring. The effectiveness of this invention may be improved particularly if the drillstring surface power management control system is augmented by selected data indicating the real-time status of downhole rotary conditions and which is transmitted in a recognizable format from the downhole to the surface location. It is a goal of this invention to enable a reduction of and, dependent upon the severity of the borehole condition, potentially to eliminate stick-slip.
Stick-slip constituted a further constraint in the entire prior art examples. The complexity of stick-slip is such that any of the following may have an effect on the magnitude of stick-slip: borehole inclination, hole-diameter, drillpipe diameter, BHA length and component configuration, bit type, bit gauge, bit cutter types, formation type, formation bedding planes and drilling fluids. Stick-slip is most noticeable during drilling, i.e. has a comparatively low magnitude when rotating off bottom and it is the interaction of bit with the formation which apparently contributes heavily to the largest element of stick-slip.
Van Steenwyck, in 2003 [U.S. Pat. No. 6,651,496], “INERTIALLY STABILIZED MAGNETOMETER MEASURING APPARATUS FOR USE IN A BOREHOLE ROTARY ENVIRONMENT”, proposes a device for reducing the effect of stick-slip on instruments which are rotationally co-located within a drillstring. (Ibid. FIGS. 1(a) through 1(d) and provide diagrammatic examples of the influence of stick-slip on sensor output for sensors which are co-located within a collar mechanism which is being subjected to stick-slip forces.
[McLOUGHLIN, U.S. Pat. No. 6,847,304] “APPARATUS AND METHOD FOR TRANSMITTING INFORMATION TO AND COMMUNICATING WITH A DOWNHOLE DEVICE, proposed the superimposition of magnetic field(s) over the prevailing geomagnetic field, and constructed a means of transferring signal from surface, via the rotating drillstring, to a downhole electromechanical sub-assembly which incorporated a non-rotating portion as a component of a three-dimensional rotary steerable drilling device.
Acknowledging and utilizing the increases in downhole electronic sampling and processing power which had occurred since the ENGELDER Patent, McLoughlin proposed a frequency modulated approach to data transmission. During the prototyping phase of the downhole device explained in U.S. Pat. No. 5,979,570 to McLoughlin et al, SURFACE CONTROLLED WELLBORE DIRECTIONAL STEERING TOOL, industry professionals expressed concern that the communications methodology which is described in U.S. Pat. No. 6,847,304 to MCLOUGHLIN would be ineffective when communicating with a device located at the distal end of the drilling assembly.
Apocryphal reasons for this belief centered around drillstring properties; PAVONE, U.S. Pat. No. 5,507,353 METHOD AND SYSTEM FOR CONTROLLING THE ROTARY SPEED STABILITY OF A DRILL BIT notes “because the drill collar assembly is very stiff against torsional strain there is practically no speed difference between (the drill collars) and the drill-bit.”
The same cannot, however, be said for the drill pipe string, which typically comprises the greater part of the total length of a drilling assembly and which stretches between the surface of the Earth and the drill collar sub assembly. drill-pipe is highly flexible and exhibits torsional harmonic vibration, or oscillatory behavior.
Drill pipe behavior under torsion is unarguably complex; DOMINICK, U.S. Pat. No. 6,065,332, METHOD AND APPARATUS FOR SENSING AND DISPLAYING TORSIONAL VIBRATION, offers a concise explanation of drillpipe behavior and the forces acting thereon:                “During drilling operations, a drillstring is subjected to axial, lateral, and torsional loads stemming from a variety of sources. In the context of a rotating drillstring, torsional loads are imparted to the drillstring by the rotary table, which rotates the drillstring, and by the interference between the drillstring and the wellbore. Axial loads act on the drillstring as a result of the successive impacts of the drill bit on the cutting face, and as a result of the irregular feed rate of the drillstring by the driller. The result of this multitude of forces applied to the drillstring is a plurality of vibrations introduced into the drillstring. The particular mode of vibration will depend on the type of load applied. For example, variations in the torque applied to the drillstring will result in a torsional vibration of the drillstring.        At the surface, torsional vibration in the drillstring appears as regular, periodic cycling of the rotary table torque. The torsional oscillations usually occur at a frequency that is close to a fundamental torsional mode of the drillstring, which depends primarily on the drill pipe length and size, and the mass of the bottom hole assembly. (BHA)”        
When it is considered that any drilling assembly has multiple vibration inducing variables acting thereon it is unsurprising that reservations were expressed as to the ability of the McLOUGHLIN communications method to adapt to a wide variety of drilling scenarios. However the simple observation behind this patent concept was that if, at the surface of the earth, a million revolutions are input into the drillstring and subsequently a million revolutions are not delivered to the distal end of the drilling assembly, then communications will not be the issue—there will be more pressing problems with the drilling assembly. Largely then, the effectiveness of this method of communications protocol is determined by ‘when’ the revolutions which are input at the surface of the earth are delivered to devices located at the distal end of the drilling assembly, i.e. timing.
In view of the novelty of the communications format, the lack of field experience and the criticality of the application, it was determined that optimal chances of success would occur if data sets were separated, one from the other by “null” data sets, otherwise referred to as “data-gaps”. Gaps were defined by reducing the drilling RPM substantially, either to zero, i.e. non-drilling or below a rotational threshold speed at which drilling would be severely compromised. In practical applications of this patent, all communications protocols were designed with ‘null’ interpolation as illustrated in FIGS. 3A and 3B of U.S. Pat. No. 6,847,304. This format is still in use today.
Despite successes with the McLOUGHLIN method of rotary communications, this approach, as with earlier devices, leaves the drilling process compromised as rotation has to stop on at least one occasion per data (point) transmission sequence or “data set” in order to provide a baseline or relational marker for the data transmission to occur.
With all the examples of prior art cited herein, it is evident that a more sophisticated or detailed data downlink will result in a longer transmission time with a corresponding increase in the potential for data corruption or transmission failure between the surface and the distal components located in the bottom hole assembly. The instant method and system proposes an improved methodology for increasing the range of data transmitted from the surface of the earth to sensors located at the distal component of the drilling assembly without increasing the risk of transmitting corrupted data.
The McLoughlin prior art considered that microprocessor speeds were sufficient to overcome the limitations in earlier devices and that the actual drillstring RPM could be monitored by sensors which had higher data acquisition rates than had been available in the past, such that the actual instantaneous RPM could be monitored and used as an integer in the transmission of data to the downhole location.
Field experience of this mechanism and methodology proved that the microprocessor speed was sufficient to keep up with drillstring RPM in excess of 300 RPM. Field experience also proved that, even with severe stick-slip, the device was capable of transmitting RPM to a very small window of accuracy, such that the required toolface accuracy could be transmitted within less than 3° tolerance, corresponding to an ability to read within +/−2 RPM. In field trials and in commercial deployment, this format, incorporating null data blocks was always used, typically with a reported 2σ or 95% first time success ratio.
The mechanism was also able to compensate for stick-slip by monitoring real-time revolutions such that the revolutions were measured against a time baseline and averaged over a given, pre-determined period. Given the requirement for absolute certainty in the application of three-dimensional direction trajectory control, a preamble was added to the transmission sequence to ensure that no command sequences were inadvertently transmitted to the downhole device.
The invention was limited in scope as the preferred downhole target device was a non-rotating stabilizer specified in McLOUGHLIN et al U.S. Pat. No. 5,979,570 SURFACE CONTROLLED WELLBORE STEERING TOOL and further in U.S. Pat. No. 6,808,027, WELLBORE DIRECTIONAL STEERING TOOL. This constrained the practical application of U.S. Pat. No 6,847,304, as its application was limited to devices which had non-rotating sleeve characteristics. The device was, additionally, constrained in that it was unidirectional in nature and did not contemplate confirmation of the transmission receipt from the downhole device. The lack of a confirmation response meant that the talkdown protocol had to be infallible in order to gain commercial acceptance. The critical requirement for absolute certainty of data transfer from the surface location to downhole meant that sample times were extended which provided constraints to the economic viability of the method and device in terms of the amount of data or data density which could effectively be transmitted from the surface of the earth to the downhole device.
Prior art, individually and collectively, thus envisaged simple, single phase, transmissions, incorporating periods of ‘zero’ rotation, even when frequency modulation was contemplated.
Thus, there remains a need to provide an adaptive system to communicate with devices located at the distal end of the drilling assembly that is devoid of “zero” rotation time periods and effective when stick-slip and other complications in the drilling process are present.